Variable resistor controls in the prior art are most commonly of the three-terminal type which can be used as either potentiometers or rheostats. When such a device is used as a rheostat, only two of the three terminals are employed in making connections within an electrical circuit. An example of such a variable resistor is disclosed in Casey et al, U.S. Pat. No. 3,657,688, issued Apr. 18, 1972.
A type of variable resistor which is designed for infrequent adjustments, sometimes only a single adjustment, is referred to as a "trimming resistor" or simply a "trimmer." Several examples of such trimmers are disclosed in Denes, U.S. Pat. No. 4,100,525, issued July 11, 1978. FIGS. 1-3, each show a two-terminal device that may be set at the beginning of its operating life to obtain a desired resistance for that operating life. A second adjustment can be made, if desired, but only within a narrowed range of values determined by the first adjustment. Trimmers are usually mounted on a circuit board, and are not usually accessible on a control panel. Trimmers, like front panel controls, are available as three-terminal devices which can be used either as potentiometers or rheostats. A three-terminal trimmer is shown in FIG. 4 of U.S. Pat. No. 4,100,525 mentioned above.
It is common practice for manufacturers of electrical apparatus to stock a single trimmer in each electrical category for use as either a potentiometer or a rheostat. While a device that is to be used exclusively as a rheostat might be made at slightly lesser cost than a comparable device which must also function as a potentiometer, the potential administrative cost of stocking two devices, including the cost of stocking added inventory, has generally outweighed the potential cost savings. Thus, it is not enough for a trimmer that functions only as a rheostat to be slightly less expensive than a trimmer that can function as either a rheostat or a potentiometer; such a rheostat must clearly economize and eliminate nonessential details.